1. Field of the Invention
The present invention relates to the field of corneal topographers, and particularly to corneal topographers determining cornea topography from an image of a placido disc reflected from an eye and observed through the center of the disc.
2. Description of Related Art
Corneal topographers are instruments that measure the anterior surface of the cornea. The most commonly used topographer is the videokeratograph, which is based on what is known as a placido disc design. A placido disc consists of a series of concentric circles of alternating black and white rings or other colors. Originally, the placido disc rings were positioned in a flat plane, but now the rings usually have a three dimensional configuration. The disc is illuminated, typically from the rear and placed in front of the cornea to be measured.
A reflection of the rings is obtained from the corneal surface when observed, such as by viewing, photographing or videographing the corneal image through a hole in the center of the disc. A principal deficiency of this method is that the central hole through which the target reflection is measured contains no object that is reflected from the central cornea, and hence this corneal area cannot be measured. Additionally, areas immediately adjacent to the central cornea also cannot be measured because the distance is too small to provide a reliable measurement based upon the optical laws of object-to-image relationships. There is therefore a need for a method of accurately measuring the central zone of a cornea using a placido disc based corneal topographer. As used herein, the central zone of the cornea is a region containing the entrance pupil.